1. Field of the Invention
The invention relates to improving the performance of plate type falling film evaporators, and more particularly to an arrangement for pre-heating of feed liquor while cooling vent gases.
2. Description of the Prior Art
Falling film evaporators in which a liquid flows down the surfaces of heating elements which consist of pairs of plates joined at their peripheries are widely used. Such an evaporator is described in U.S. Pat. No. 3,366,158, issued Jan. 30, 1968. A selective condenser which employs plate type heat exchange elements is described in U.S. Pat. No. 4,216,002, issued Aug. 5, 1980.
Evaporators of the plate type, and evaporator systems generally, are subject to radiation losses and venting losses. Radiation losses can be reduced by use of insulation. Venting loses can be reduced by subcooling the vent gases before they exit the evaporator.
In some types of evaporators subcooling has been accomplished to some extent by the use of some form of internal liquor preheater/vent gas cooler arrangement and/or by the use of external heaters, but such arrangements have not been successfully employed in plate type falling film evaporators, because incoming liquor exchanges heat directly with vapor generated within the evaporator more efficiently than by indirect heat transfer with heating medium within the plate elements. This process dissipates the available temperature differential and driving force.